The field is lubrication management for two-stroke cycle engines. More specifically the application relates to implementation of a wristpin oil pressure recovery device for pistons of a two-stroke cycle, opposed-piston engine.
Wristpins in reciprocating engines must be lubricated to mitigate the risk of highly loaded asperity contact in the joint. If asperity contact in the joint is sustained at high loads, excessive friction, wear and even catastrophic failure is possible. The applied load that causes this asperity contact is constantly changing as engine speed and load change.
In some aspects of two-stroke cycle opposed-piston engine operation, the nature of the cycle presents two distinct threats to wristpin durability: continuous compression loading and oil pressure variation.
Continuous compression loading results because load reversal on the wristpin bearings of a two-stroke engine may never occur during the normal speed and load range operation of the engine. During operation of a two-cycle engine, a combustion event occurs every cycle and there is nearly always a gas pressure loading the crown of a piston near top center (TC), which, even at high piston speeds, is still greater than the inertial force of the piston assembly on a wristpin bearing. At the other end of the cycle, at bottom center (BC), the inertial force of the piston assembly keeps the bearing loaded as well. As a result, the bearing is nearly always under positive load throughout the cycle, and it is difficult to replenish it with oil. Furthermore, given limited angular oscillation of the bearing, oil introduced between the bearing surfaces does not completely fill the bearing. Eventually the bearing begins to operate in a boundary layer lubrication mode which leads to excess friction, wear, and then bearing failure.
Solutions to the first problem include bearing constructions that cause separation of bearing parts in response to bearing rotation. One such solution is disclosed in related U.S. application Ser. No. 13/776,656: wristpins coupling the pistons of an opposed-piston engine are constructed with rocking journal bearings that provide biaxial rotation of bearing parts, which separates the parts to allow introduction of oil between the bearing surfaces. This bearing construction includes a reservoir in the rocking journal that acts as an accumulator to receive and maintain a volume of pressurized oil that is delivered to the bearing parts via outlet passages through the journal. An inlet passage in the journal for delivering oil to the accumulator is fed from a high-pressure oil passage in the associated connecting rod. Pressurized oil is transported to the oil passages of the connecting rods from a main oil gallery in an engine block. Here, the second problem becomes apparent.
As a pair of pistons move in opposition in a cylinder bore, the pressurized oil fed to their respective wristpins undergoes inertial loading that is most pronounced when the pistons change direction in the reversal zones at their top center (TC) and bottom center (BC) locations. Given the direction of motion from TC to BC during a power stroke, the inertial load can cause the oil pressure in the connecting rod oil passages to drop below a minimum level for effective wristpin lubrication as the pistons reverse direction at BC. Providing adequate oil pressure to lubricate the wristpins throughout the operating cycle of an opposed-piston engine, especially in the face of non-reversing loads, may require that the supply pressure to the main oil gallery increase with engine speed to overcome inertial forces on the oil column in the connecting rod. Typically, the main oil gallery is fed from a positive pressure pump, and it is possible to control the pump so as to vary the supply pressure with the speed of the engine. However, increasing engine-wide oil pressure solely for wristpin lubrication, as the speed of the engine increases, may result in oil pressure in excess of that required for the rest of the engine lubrication system. This will result in higher parasitic loads for the lubrication system and a higher friction-mean-effective-pressure (FMEP) for the engine.
Accordingly, there is a need for maintaining oil pressure in the oil reservoir of the wristpin of a two-stroke cycle, opposed-piston engine during engine operation. It is particularly desirable that the oil pressure be maintained at levels that guarantee effective lubrication throughout the operating cycle, at all engine speeds, without imposing excessive pumping losses on engine performance.